There are many researches being conducted recently on the Orthogonal Frequency Division Multiple Access (OFDMA) and Single Carrier-Frequency Division multiple Access (SC-FDMA) in the cellular communication field. Such a multiple access technology is used to allocate and manage the time-frequency resources for data and/or control information transmission to and from multiple users without overlapping from each other, i.e. orthogonally. This makes possible to discriminate among per-user data and control informations.
One of the significant features of the cellular communication system is to support scalable bandwidth for providing high speed wireless data service. For example, the Long Term Evolution (LTE) system is capable of supporting various bandwidths, e.g. 20/15/5/3/1.4 Mhz. The mobile carriers can provide their services with one of the available bandwidths. A User Equipment (UE) can operate with various capabilities of minimum 1.4 MHz bandwidth up to 20 MHz bandwidth. Meanwhile, the LTE-Advanced (hereinafter, called LTE-A) system can support high data rate transmission over a wide bandwidth up to 100 MHz for a single UE with carrier aggregation (CA) technique.
In order to support the high data rate transmission, the LTE-A system requires the bandwidth wider than that of the LTE system while preserving backward compatibility to the legacy systems for supporting the LTE UEs. For the backward compatibility, the system bandwidth of the LTE-A system is divided into a plurality of subbands or component carriers (CC) that can be used for transmission/reception of LTE UEs and aggregated for the high data rate transmission of the LTE-A system with the transmission/reception process of the legacy LTE system per component carrier.
Each component carrier or cell can be categorized into one of primary cell and secondary cell according to its usage or significance. From the view point of the UE, only one primary cell exists with other secondary cells. In the current LTE-A system, the uplink control channel can be transmitted in the primary cell while uplink data channel can be transmitted in both the primary and secondary cells.
The scheduling information for the data transmitted on the component carriers is transmitted to the UE in Downlink Control Information (DCI). The DCI is generated in different DCI format according to whether scheduling information is of uplink or downlink, whether the DCI is compact DCI, whether spatial multiplexing with multiple antennas is applied, and whether the DCI is the power control DCI. For example, the DCI format 1 for the control information about downlink data to which Multiple Input Multiple Output (MIMO) is not applied is composed of the following control informations.                Resource allocation type 0/1 flag: It notifies the UE of whether the resource allocation type is type 0 or type 1. Here, type 0 indicates resource allocation in unit of resource block group (RBG) in bitmap method. In LTE and LTE-A systems, the basic scheduling unit is resource block (RB) representing time and frequency resource, and RBG is composed of a plurality of RBs and basic scheduling unit of in type 0. Type 1 indicates allocation of specific RB in RBG.        Resource block assignment: It notifies the UE of RB allocated for data transmission. At this time, the resource expressed according to the system bandwidth and resource allocation scheme is determined.        Modulation and coding scheme: It notifies the UE of modulation scheme and coding rate applied for data transmission.        HARQ process number: it notifies the UE of HARQ process number.        New data indicator: It notifies the UE of whether the transmission is HARQ initial transmission or retransmission.        Redundancy version: It notifies the UE of redundancy version of HARQ.        TPC command for PUCCH: It notifies the UE of power control command for Physical Uplink Control Channel (PUCCH) as uplink control channel.        
In TDD communication system, the downlink and uplink share the same frequency and discriminated from each other in time domain. In LTE TDD mode, the downlink and uplink signals are discriminated from each other per subframe. According to the traffic loads in uplink and downlink, the uplink/downlink subframes are assigned symmetrically or asymmetrically in time domain. This means that one of the downlink and uplink can be assigned more subframes than the other. In LTE, a subframe has the length of 1 ms, and 10 subframes form a radio frame.
TABLE 1Uplink-downlinkSubframe numberconfiguration01234567890DSUUUDSUUU1DSUUDDSUUD2DSUDDDSUDD3DSUUUDDDDD4DSUUDDDDDD5DSUDDDDDDD6DSUUUDSUUD
Table 1 shows the TDD UL-DL configurations defined in LTE standard. In table 1, ‘D’ denotes the subframe configured for downlink transmission, and ‘U’ denotes the subframe configured for uplink transmission. ‘S’ denotes the Special subframe composed of Downlink Pilot Time Slot (DwPTS), Guard Period (GP), and Uplink Pilot Time Slot (UpPTS). The DwPTS can be used for transmitting control information in downlink like a normal subframe and even downlink data if it is elongated long enough according to the configuration of the special subframe.
The GP is a period for switching from downlink to uplink and its length is determined according to the network configuration. The UpPTS is used for transmitting UE's Sounding Reference Signal (SRS) for uplink channel state estimation or Random Access Channel for UE's random access.
In an exemplary case of TDD UL-DL configuration #6, subframes #0, #5, and #9 are configured for downlink data and control information transmission, subframes #2, #3, #4, #7, and #8 are configured for uplink data and control information transmission. The special subframes #1 and #6 can be used for control information or data transmission in downlink and SRS or RACH in uplink.
In TDD system, since the downlink or uplink transmission is allowed for specific time duration, it is necessary to define detailed timing relationship between uplink and downlink physical channels correlated such as control channel for data scheduling, data channel to be scheduled, and HARQ ACK/NACK channel corresponding to the data channel.
The uplink/downlink timing relationship between the Physical Shared Channel (PDSCH) as downlink data transmission channel and the Physical Uplink Control Channel as physical channel for transmitting uplink HARQ ACK/NACK corresponding to the PDSCH or Physical Uplink Shared Channel (PUSCH).
If PDSCH is received from the eNB in (n−k)th subframe, the UE sends uplink HARQ ACK/NACK for the PDSCH in nth uplink subframe. Here, k denotes an element of a set K which is defined as illustrated in table 2.
TABLE 2UL-DLSubframe nConfiguration01234567890——6—4——6—41——7, 64———7, 64—2——8, 7, 4, 6————8, 7, 4, 6——3——7, 6, 116, 55, 4—————4——12, 8, 7, 116, 5, 4, 7——————5——13, 12, 9, 8, 7, 5, 4,———————11, 66——775——77—
Table 3 shows the UL-DL configurations of subframes carrying HARQ ACK/NACK rearranged according the definition of table 2 when PDSCH is transmitted downlink subframe (D) or special subframes (S)n.
TABLE 3Subframe n UL-DLConfiguration01234567890D4S6UUUD4S6UUU1D7S6UUD4D7S6UUD42D7S6UD4D8D7S6UD4D83D4S11UUUD7D6D6D5D54D12S11UUD8D7D7D6D5D45D12S11UD9D8D7D6D5D4D136D7S7UUUD7S7UUD5
FIG. 1 is a diagram illustrating a principle of transmitting uplink HARQ ACK/NACK in the conventional method. FIG. 1 shows which frame is used for transmitting the uplink HARQ ACK/NACK corresponding to the PDSCH transmitted in downlink or special subframe in TDD UL-DL configuration #6 according to the definition in table 3. A description is made of table 3 with reference to FIG. 1.
For example, the UE transmits HARQ ACK/NACK 103 in the subframe #7 of ith radio frame in response to the PDSCH 101 transmitted by the eNB in the subframe #0 of the ith radio frame. At this time, the DCI including the scheduling information about the PDSCH 101 is transmitted in the PDCCH of the same subframe carrying the PDSCH 101. In another example, the UE transmits the uplink HARQ ACK/NACK 107 corresponding to the PDSCH 105, which is transmitted by the eNB in the subframe #9 of the ith radio subframe, in the subframe #4 of the (i+1)th radio frame. Likewise, the DCI including the scheduling information about PDSCH 105 is transmitted through PDCCH of the same subframe carrying the PDSCH 105.
In the LTE system, an asynchronous HARQ scheme having unfixed data retransmission time point. That is, when HARQ NACK feedback is received from the UE in response to the data of HARQ initial transmission, the eNB determines the HARQ retransmission time point freely according to the scheduling operation. The UE decodes the received data and buffers the erroneous HARQ data to be combined with next HARQ retransmission data. In order to maintain the reception buffer capacity to a certain limit, the maximum number of downlink HARQ processes is defined per TDD UL-DL configuration as shown in table 4. One HARQ process maps to one subframe in time domain.
Table 4
TABLE 4TDD UL/DL configurationMaximum number of HARQ processes04172103941251566
Referring to the example of FIG. 1, the UE decodes the PDSCH 101 transmitted by the eNB in the subframe #0 of ith radio frame. If the decoding result is erroneous, the UE transmits an HARQ NACK 103 in the subframe #7 of ith subframe. Upon receipt of the HARQ NACK, the eNB transmits PDSCH 109 including the retransmission data corresponding to PDSCH 101 along with PDCCH. FIG. 1 shows an exemplary case where the maximum number of downlink HARQ processes is set to 6 according to TDD UL-DL configuration #6 of table 4 such that the retransmission data is transmitted in the subframe #1 of (i+1)th radio frame. That is, there are 6 downlink HARQ processes 111, 112, 113, 114, 115, and 116 between the initial transmission PDSCH 101 and retransmission PDSCH 109.
In the LTE system, the synchronous HARQ scheme having fixed data transmission time point is adopted in uplink unlike the downlink HARQ. That is, the uplink/downlink timing relationship among Physical Uplink Shared Channel (PUSCH), PUCCH preceding PUSCH, and Physical Hybrid Indicator Channel (PHICH) carrying downlink HARQ ACK/NACK corresponding to the PUSCH are fixed according to the following rule.
If the PDCCH including uplink scheduling control information or PHICH carrying the downlink HARQ ACK/NACK which is transmitted by the eNB at subframe n is received, the UE transmits uplink data corresponding to the control information received at (n+k)th subframe on PUSCH. Here, k determined by referencing table 5.
TABLE 5TDD UL/DL subframe number nDLConfiguration01234567890464616464244344444454677775
The UE receives PHICH carrying downlink HARQ ACK/NACK from the eNB at the ith subframe. At this time, the PHICH corresponds to the PUSCH transmitted by the UE at (i−k)th subframe. Here, k is determined by referencing table 6.
TABLE 6TDD UL/DL subframe number iDLConfiguration01234567890747414646266366646656664746
FIG. 2 is a diagram illustrating a principle of transmitting uplink PUSCH in the conventional method. FIG. 2 shows which subframe is used for transmitting the PUSCH corresponding to the PDCCH or PHICH transmitted in downlink or special subframe in TDD UL-DL configuration #1 according to the definition in tables 5 and 6.
For example, the eNB transmits PDCCH or PHICH 201 at subframe #1 of ith radio frame. In reply, The UE transmits PUSCH 203 corresponding to the PDCCH or PHICH 201 at the subframe #7 of the ith radio frame. The eNB transmits the PHICH 205 corresponding to the PUSCH at the subframe #1 of the (i+1)th radio frame.
In another exemplary case, the eNB transmits the PDCCH or PHICH 207 to the UE at the subframe #6 of the ith radio frame. The UE transmits the PUSCH 209 corresponding to the PDCCH or PHICH 207 at the subframe #2 of (i+1)th radio frame. The eNB transmits the PHICH 211 corresponding to the PUSCH 209 to the UE at the subframe #6 of the (i+1)th radio frame.
In the LTE TDD system, the downlink transmission of PDCCH or PHICH corresponding to PUSCH may be limited to specific downlink subframes. Accordingly, it is possible to secure the least transmission/reception processing time of the eNB and UE. For example, in case of the TDD UL-DL configuration #1 of FIG. 2, the PDCCH for scheduling PUSCH or PHICH corresponding to the PUSCH is not transmitted at the subframes #0 and #5.